THE CURRENT STATE OF POST-COLONIAL LANDSCAPE DISTURBANCE TO THE POTOMAC RIVER AND CHESAPEAKE BAY

The ecological health of Chesapeake Bay deteriorated following colonial American settlement, over-fishing, and regional deforestation in the 18^th–19^th centuries. Modern efforts to restore the ecological health of Chesapeake Bay have, in part, called for a reduction of sediment flux from the Bay’s contributing tributaries, of which the Potomac River is the largest. Through this analysis of landscape dynamics within the Potomac River watershed, we find that modern sediment yields (n = 10) remain higher than pre-disturbance erosion rates derived from in-situ produced cosmogenic ¹⁰Be (¹⁰Be_i; avg. = 29.6 ± 14.1 Mg km^-2 yr^-1; n = 62). Cosmogenically determined rates are consistent with model-based regulatory limits for the Total Maximum Daily Level of sediment flux from the Potomac River to Chesapeake Bay (24.3 Mg km^-2 yr^-1). Additionally, this study provides the opportunity to test recently-developed methods for deriving denudation rates from measurements of meteoric ¹⁰Be (¹⁰Be_m) normalized to measurements of the reactive phase of naturally-occurring ⁹Be (⁹Be_reac), which is weathered from parent bedrock minerals. In the Potomac River basin, we find that the average ¹⁰Be_m/⁹Be_reac denudation rate (avg. = 40.0 ± 21.7 Mg km^-2 yr^-1; n = 56) is greater, though within the uncertainty of the average ¹⁰Be_i erosion rate. When compared basin-by-basin, however, ¹⁰Be_m/⁹Be_reac denudation rates do not replicate ¹⁰Be_i erosion rates well. We interpret the disparity between ¹⁰Be_i erosion rates and ¹⁰Be_m/⁹Be_reac denudation rates as reflecting lingering effects of post-colonial landscape disturbance. We suggest that our ¹⁰Be_m samples do not contain the high concentrations of ¹⁰Be_m associated with pre-disturbance topsoil, which was eroded following deforestation and is now likely stored behind crumbling mill dams or in the tidal reaches of the Potomac River downstream from Washington, DC. The possibility remains that had post-colonial landscape change not occurred in the Potomac River basin, ¹⁰Be_m/⁹Be_reac denudation rates would more consistently reflect ¹⁰Be_i erosion rates.